The present invention relates to a method for coating a belt-shaped flexible support of plastic film, paper, metal foil, or the like (hereinafter referred to as "a web") with a coating solution such as a photographing photo-sensitive solution, magnetic solution, surface protective solution, base coating solution, lubricant solution or the like, and to a coating apparatus for practicing the coating method.
A variety of methods have been proposed in the art to apply a coating solution to a web which is being run.
One such coating method employs a doctor blade. In this method, after the web is sufficiently coated with the coating solution, excess coating solution is scraped off the web with the doctor blade, and the coating solution remaining on the web is smoothed to form a desired film layer thereon. A coating apparatus for practicing the method is simpler in construction than an extrusion type coating apparatus. In addition, the coating operation is simple, and a thin film can be formed on the web at a high speed.
One example of the coating method using a doctor blade is disclosed in Japanese Patent No. 62-4189.
In the method, the doctor blade is provided with a backup roller. The backup roller is rigid, and is confronted with the doctor blade. Hence, foreign matter on the web is liable to be caught in the gap between the doctor blade and the web. The foreign matter thus caught may form stripes on the web, and at worst may break the web. The coating rate is controlled by adjusting the gap between the end of the doctor blade and the cylindrical wall of the backup roller. For instance, in forming a film layer on the web at a coating rate of 10 cc/m.sup.2 or less, it is necessary to set the gap to 20 .mu.m or less, with the result that the above-described problem will not take place often. However, it is very difficult to set the gap to an extremely small value such as 20 .mu.m. Furthermore, oscillation of the backup roller greatly affects the accuracy of the gap, thus varying the film thickness both in the direction of width of the web and in the direction of movement of the web.
In a conventional dust removing method of this type, the doctor blade employed is higher in dust removing effect since it is sharper edged, and the object of such a doctor blade is to remove, as much as possible, coating solution from the web. Thus, the dust-removing doctor blade does not function as the
doctor blades of the above-described coating method. Specifically, the doctor blade functions only as a dust removing member. However, the conventional arrangement of the dust-removing doctor blade employed in a coating apparatus is not acceptable as means for stably controlling the thickness of a film layer formed on the web. Thus, the doctor blade is still disadvantageous. Research on this difficulty has not been sufficiently conducted in the art.